Some of the major symptoms of plant diseases are as follows:
A disease manifests itself in the form of some typical external and internal changes in the host plant.
Such visible changes, abnormalities or signs which serve to recognize the disease in the lost plant are called symptoms of the disease.
A diseased plant can easily be distinguished from a normal healthy one on the basis of a symptom. The symptoms provide clues to find out the nature of the disease and the casual agent operating on the host. All the visible symptoms are collectively called syndrome.
The symptoms of plant diseases are of following 4 types:
(iv) Gummosis and exudations.
(I) Necrotic symptoms:
Death of the host cells, tissues and organs induced by a pathogen is called necrosis. The necrotic areas are called lesions.
The various necrotic symptoms are:
(a) Streaks or strips:
These are elongated narrow lesions.
These are minute circular or sub-circular lesions of various colours such as brown, white, dark, orange or red e.g. leaf spot in the blight of tea due to Glomerella cingulata and Fungal members of Phacidiales or Doihideales cause raised black spots called tar-spots, Cystopus candidus cause white spots in the cabbage. In some cases, the dead tissue of leaf snots shed, leaving circular perforations called shot-holes.
It involves rapid discoloration followed by death of a plant organ which gives a burnt appearance, e.g. Blight ‘of Wheat, maize.
These are spreading sunken lesions in the stems, surrounded by raised margin. When canker encircles the entire branch, cause the death of distal part. This happens in Pink disease of rubber caused by Pellicularia salmoni color or in apple trees attacked by Physalospora mutila.
The affected tissues die and decompose to a great extent. For example, on the basis of organization involved, they are root rot, stem rot, leaf rot, bud rot, fruit rot etc. On the basis of type of decomposition they are soft rot, dry rot, wet rot and black rot.
(f) Damping off:
The stem of seedling attacked near the ground level so that the seedlings collapse. Pythium and Rhizoctonia are important fungi causing damping-off of seedlings in chili, tobacco, mustard, tomato, castor, cucurbits etc.
(g) Burn, Scald or Scorch:
Brownish lesions in succulent organs due to high temperature.
Progressive death of the affected part from the tip downwards.
The sunken lesions with raised margins on fruits, pods, chilies etc.
(II) Hypoplasia (Atrophy):
This is a symptom of many diseases where the growth of plants arrest causing stunting or dwarfing e.g. Suppressed floral buds in mustard is due to Peronospora brassicae. In some cases internodes fail to elongate causing the growing of foliage which gives a rosette forms. This is called Resetting, e.g., groundnut rosettes (a viral disease).
(III) Hyperplasia and Hypertrophy:
Excessive growth of host tissues caused due to hyperplasia and hypertrophy. In hyperplasia the overgrowth is due to increased cell division and cell number e.g. corn, smut. But in hypertrophy the overgrowth of organ is due to increase in size of individual cells, e.g. corn galls, witches brooms of Cherry etc. In mustard plants Cystopus candidus causes hypertrophy of floral parts.
Some symptoms of abnormal growth are:
These are abnormal fleshy or woody outgrowths, when small called warts or tubercles, when large called knots, e.g. Potato wart caused by Synchytrium endobioticum, mustard root- galls by Urocystis brassicae.
Those are wart, like swellings of epidermal cells of leaves due to environmental factors,
Leaves become puckered or crinkled, e.g. peach leaf-curl due to Taphrina defeormans.
(d) Hairy root:
These are cluster of fibrous roots that appear like root hairs.
Superficial growth on fruit.
(IV) Discolouration Symptoms:
The yellowing of green parts due to lack of light is called etiolation. But, when it happens due to mineral deficiency, low temperature or infection of pathogens is called chlorosis. When leaves appear transparent due to lack of any pigment is called albinism. Colour conversion to red, purple or orange is called chromosis. Sometimes due to viral or fungal infection yellow and green patches found irregularly called mosaic. The discolouration surrounding leaf veins is called vein banding and the discolouration along leaf veins is called vein clearing.
(V) Gummosis and Exudations:
The production of a clear, amber-colour exudates on the surface of the affected parts to a plant, which later sets into a solid mass insoluble in water, is known as gummosis. Gummosis is common in cherry, peach and citrus trees. Reddish exudate is a common feature of me stems bleeding disease of coconuts caused by Ceratostomella paradoxa.
(VI) Other Symptoms:
i. Phyliody or Virecence:
Transformation of floral organs to vegetative organs, e.g. in peal millet flowers convert into leave, due to downy mildew.
Coloured pustules on host surface due to fungal spores.
These are the affected areas releasing charcual-like dusty mass of spores.
These are the coloured superficial patches on the host surface due to fungal infection. When the superficial patches appear cottony or downy called downy mudews and when dusty or powdery called powdery mudew appears.
It is withering or drooping of whole plant due to loss of turgidity. It generally caused due to excessive transpiration, injuring to root system, toxins of pathogens etc.
vi. Witches brooms:
The woody branches of infected tree become swollen from which upward turned shoots and small leaves arise which give broom – like appearance e.g. witches brooms of deodar plant caused by Peridermium cedri.